The background of the present invention is described herein in the context of subscription television systems, such as cable television and direct broadcast satellite (DBS) systems, that distribute a variety of program services to subscribers, but the invention is by no means limited thereto except as expressly set forth in the accompanying claims.
In the subscription television industry, "programmers" produce "programs" for distribution to various remote locations. A "program" may consist of video, audio and other related services, such as closed-captioning and teletext services. A single programmer may wish to supply many programs and services. Typically, a programmer will supply these services via satellite to individual subscribers (i.e., DBS subscribers) and/or cable television operators. In the case of cable television operators, the services transmitted via satellite are received at the operator's "cable head-end" installations. A cable operator typically receives programs and other services from many programmers and then selects the programs/services it wishes to distribute to its subscribers. In addition, a cable operator may insert locally produced services at the cable-head end. The selected services and locally produced services are then transmitted to the individual subscribers via a coaxial cable distribution network. In the case of DBS subscribers, each subscriber is capable of receiving a satellite down-link from the programmers directly.
In the past, subscription television systems, including cable and DBS systems, have operated in the analog domain. Recently, however, the subscription television industry has begun to move toward all digital systems wherein prior to transmission, all analog signals are converted to digital signals. Digital signal transmission offers the advantage that digital data can be processed at both the transmission and reception ends to improve picture quality. In addition, digital data compression techniques have been developed that achieve high signal compression ratios. Digital compression allows a larger number of individual services to be transmitted within a fixed bandwidth. Bandwidth limitations are imposed by both satellite transponders and coaxial cable distribution networks, and therefore, digital compression is extremely advantageous.
Several problems and concerns arise when one considers an all digital program services delivery system. First, there are a wide range of digital service rates in use throughout the industry. For example, digital video service rates differ from digital audio service rates. Digital video rates themselves can range from 17 Mbps (High Definition Television--HDTV) to as low as 1.544 Mbps (telecommunications standard T1). A problem arises when a source programmer wants to transmit multiple digital services having different data rates. There is a need, therefore, for a system that can accommodate different service data rates.
Second, many digital compression algorithms produce a single compressed data stream that has a dynamically varying data rate. Most often this occurs because certain portions of the original data are redundant and the compression algorithm need only transmit a small amount of data to represent those redundant portions. For example, in a sequence of video frames, the "scene" may not change significantly from frame to frame. Rather than transmit a redundant frame, a short code can be transmitted in its place indicating that the frame is substantially the same as the previously transmitted frame. During a typical television program, there are periods where scenes change rapidly (e.g., during an action sequence) and periods where scenes are relatively constant (e.g., during an interview). Consequently, the rate of compressed data will vary dynamically throughout the program. When multiplexing a plurality of compressed digital video services into a single multiplexed data stream having a fixed overall data rate, it is desirable to dynamically allocate portions of the overall data stream to the various video services depending on the individual data rate of each service at given times. This method of dynamic allocation is know as statistical multiplexing. Statistical multiplexing ensures a more efficient use of system bandwidth. Because video and audio compression will be used widely in the subscription television industry, it is desirable for any method of transmitting multiple compressed services to support statistical multiplexing.
Another problem that arises in the subscription television industry context is that system requirements differ from programmer-to-programmer and cable operator-to-cable operator. Also, the requirements of individual programmers and cable operators change over time. For example, a programmer may initially determine that it wants to provide five video services. Later, that programmer may want to expand its offerings and provide additional video services. Having to replace system hardware, however, is undesirable. A system that allows programmers and cable operators to expand the number and type of services they provide without major hardware changes or upgrades is desirable.
The present invention provides a system and method of transmitting multiple digital services, including video, audio, closed-captioning and teletext services, that satisfies the needs described above.